Drilling/milling guide and keel cut preparation system

ABSTRACT

An instrument system, associated milling or drilling guide and method include use of a trial implant of a size corresponding to an actual implant for the intervertebral space, with a milling guide mounted on the trial implant. The milling guide includes a longitudinal guide chamber which is tapered from a forward end to a rearward end. The system also includes a cutting tool which is received in the guide chamber in order to form a cutout in an adjacent vertebra. This cutting tool includes a bearing member which pivotally engages the rearward end to form a pivot axis for the cutting tool in the tapered guide chamber. Either the milling guide can be inverted to form the cutout in the other vertebra; or the milling guide can be provided with two guide chambers, to be used with one cutting tool moved between them or two respective cutting tools.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.60/834,178, filed Jul. 31, 2006.

FIELD OF THE INVENTION

This invention relates to intervertebral implants, and morespecifically, it relates to new and improved guides, systems and methodsfor cutting a keel slot in preparation for inserting an intervertebralimplant in the intervertebral space.

BACKGROUND OF THE INVENTION

Currently, when it is necessary to completely remove a disc from betweenadjacent vertebrae, the conventional procedure is to fuse the adjacentvertebrae together. More recently, there have been importantdevelopments in the field of disc replacement, namely disc arthroplasty,which involves the insertion of an artificial intervertebral discimplant into the intervertebral space between adjacent vertebrae. Thisthen allows limited universal movement of the adjacent vertebrae withrespect to each other.

Some instruments have been developed to date for preparing anintervertebral space for receiving an artificial disc implant. Theseinclude a set of different sizes of trial implants, different ones ofwhich are inserted into a cleaned out intervertebral space until thecorrect size trial implant has been determined, thereby determining thesize of the actual disc implant to be permanently inserted. The trialimplant may have a fixed stop member in the form of a pin fixed to therear end of the trial implant and extending vertically up and down forlimiting movement of the trial implant into the intervertebral space.

Some disc implants have a raised keel on each endplate which requiresthat a cutout be formed in the vertebrae adjacent the intervertebralspace for receiving these raised keels. One known arrangement forforming these cutouts is with a chisel which can be mounted to movealong slots in the top and bottom of the selected trial implant as thechisel cuts into the adjacent vertebrae to form the cutouts.

Besides a slot made by chiseling, drilling or milling can also be used,and combinations of these procedures are possible as well. However,where a chisel cut is made using a chisel and a mallet, quite highforces are applied in direction of the cut. With drilling, lesser forcesare applied, but the drill can slip of or bend during drilling. Withmilling, a precise cut is made without high forces, but the milling toolneeds to have a certain diameter, because otherwise it will break duringmilling and consequently milling is not always possible where a longnarrow cut is required. Thus, a procedure used to perform narrow cutswithout applying high forces is desirable. Exemplary of such prior artdevices and methods are those disclosed in USPA 2004-0215198 (Marnay etal.) and USPA 2006-0064100 (Bertagnoli et al.), which are herebyincorporated by reference.

One known artificial disc implant is shown in Published Application No.WO 01/01893, published Jan. 11, 2001; and instruments for inserting sameare shown in U.S. Pat. No. 7,118,580 (Beyersdorff—or PublishedApplication No. WO 01/19295) and USPA 2004-0215198 (Marnay—or PublishedApplication No. WO 04/098380). These references are also herebyincorporated by reference.

While these known instruments and methods represent a substantialimprovement in the art, there exists a continuing need for improvementsin the field of instruments and methods for preparing an intervertebralspace for receiving an artificial intervertebral disc implant.

BRIEF SUMMARY OF THE INVENTION

The purpose of the present invention is provide new and improvedinstruments and related methods for preparing an intervertebral spacefor receiving an artificial intervertebral disc implant.

The instruments of the present invention may be used to prepare theintervertebral space at any location along the spine includingespecially the lumbar and cervical spines. However, since the cervicalvertebrae are so small relative to the lumbar vertebrae, i.e., about 20%of the area of the lumbar spine vertebrae, some instruments may be moresuited than others for the cervical spine.

At present, the intervertebral implant is normally inserted from thepatient's anterior moving towards the patient's posterior. However, itis to be understood that the implant, the instruments and the method canalso be designed and arranged to insert the implant laterally, i.e.,from the side, in which case the keels will be oriented on the implantfor such lateral movement and the cutouts in the adjacent vertebrae willbe opened toward a lateral side to receive the keel. To avoid confusionwith respect to the patient's anatomy, the invention will be describedherein with respect to more simple terminology which relates to theinstruments and methods themselves. For example, in describing theinvention, the terms “front” or “forward” mean the part of theinstrument which faces toward the vertebrae or is moving in thedirection of movement toward the vertebrae, while the words “back”,“rear” or “rearward” refer to the end of the instrument farthest fromthe vertebrae or moving away from the vertebrae. Also, in thisapplication, the words “upper” or “lower” or “uppermost” or “lowermost”or any other words describing the orientation of the intervertebralimplant or the instruments or methods associated therewith are used onlyfor convenience and are not intended to convey any limitation. Morespecifically, the parts of the implant, the instruments and/or themethods described in this application with reference to the upper partcan in fact be positioned as the superior or inferior part within thepatient's vertebrae, with the other of the two parts being the oppositepart.

It is thus an object of the present invention to provide new andimproved instruments and methods for preparing an intervertebral spacefor receiving an artificial intervertebral disc implant.

The instruments and the methods of the present invention areparticularly adapted for use with an artificial intervertebral discimplant having upper and lower parts which undergo limited universalmovement with respect to each other, with the upper and lower surfacesof the upper and lower parts engaging the adjacent vertebral surfaces.Most of the instruments and methods of the present invention are alsofor use where the implant has a keel extending from the vertebraeengaging surfaces of the implant into cutouts formed in the adjacentvertebrae.

In accordance with a first aspect of the present invention, there isprovided improved instruments and methods for inserting different sizetrial implants (until the correct trial implant has been determined) incombination with forming the cutouts in the vertebrae. The instrumentsystem includes a trial implant of a size corresponding to an actualimplant for the intervertebral space, and a milling guide mounted on thetrial implant. The milling guide includes a (or two, side by side)longitudinal guide chamber which is tapered from a forward end to arearward end. The system also includes a cutting tool which is receivedin the guide chamber in order to form a cutout in an adjacent vertebra.This cutting tool includes a bearing member which pivotally engages therearward end to form a pivot axis for the cutting tool in the taperedguide chamber. Either the milling guide can be inverted to form thecutout in the other vertebra; or the milling guide can be provided withtwo guide chambers, to be used with one cutting tool moved between themor two respective cutting tools.

In a preferred embodiment of the instrument system, the guide chamber istapered in a cranial to caudal direction. In addition, the cutting toolhas a cutting head which is conically shaped and the trial head includesa longitudinal groove adjacent the guide chamber in which the cuttinghead is receivable. This longitudinal groove is also preferably largerthan the cutting head to provide a repository for cut vertebra.

Also in a preferred embodiment, the trial implant includes an adjustablestop which engages the adjacent vertebra when the trial head is properlypositioned in the intervertebral space. Then, the milling guide includesa mounting means for movably mounting the milling guide on the trialbody to a preset position. In addition, the bearing member is movablyadjustable on the cutting tool.

In the preferred embodiment, the rearward end of the milling guideengaged by the bearing member includes a bearing mechanism so that thepivot axis does not move during cutting. In one embodiment, the bearingmechanism is a curvature of a portion of the rearward end engaged by thebearing member. In another embodiment, the bearing mechanism is amovable bearing mounted in the rearward end; which can be removablymounted if desired. In still another embodiment, the bearing mechanismincludes an elongate tube extending towards the forward end in order toprovide additional support for the cutting tool during cutting.

Also in accordance with first aspect, a milling guide and method ofusing the milling guide to produce the cutouts are provided. With themilling guide, and in the method, rotating and moving of the cuttingtool is used to produce a windshield wiper sweeping motion of the end ofthe cutting tool in order to form a cutout in an adjacent vertebra. Thiscutting is made by the longitudinal moving of a bearing member locatedon the cutting tool until the bearing member pivotally engages therearward end of the milling guide to form a pivot axis for the cuttingtool in the tapered guide chamber.

In accordance with a second aspect of the present invention, a millingguide for use with an instrument system for preparing an intervertebralspace for receiving an implant, includes a mounting means for mountingon the trial implant at a preset position and a pair of longitudinalguide chambers. These guide chamber extends from a common entrance holein a rearward end to a respective exit hole at a forward end, Thus, adrilling tool is received in the entrance end is selectively movable outof either exit hole as desired and in order to form two drilled holes inthe adjacent vertebra.

It is an object of the present invention to provide new and improvedinstruments for preparing an intervertebral space for receiving anartificial disc implant.

It is another object of the present invention to provide new andimproved methods for preparing an intervertebral space for receiving anartificial disc implant.

These and other objects of the present invention will be apparent fromthe detailed description to follow, together with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of an intervertebral implant adjacent anintervertebral space between two vertebral bodies.

FIG. 2 is a perspective view of the vertebral bodies now having keelslots provided therein.

FIG. 3 is a perspective view of an intervertebral implant partiallyinserted into the intervertebral space between two vertebral bodies.

FIG. 4 is a perspective view of the intervertebral implant fullyinserted into the intervertebral space between two vertebral bodies.

FIG. 5 is a front, side and plan perspective view of a milling system inaccordance with the present invention.

FIG. 6 is a back, side and plan perspective view of the milling systemof FIG. 5 positioned with the trial implant in an intervertebral space.

FIG. 7 is a front, side and plan perspective view of a trial implant inaccordance with the present invention.

FIG. 8 is a front, side and plan perspective view of the trial implantof FIG. 7 with an attached handle.

FIG. 9 is a front, side and plan perspective view of a milling guide inaccordance with the present invention.

FIG. 10 is a cross sectional front, side and plan perspective view ofthe milling system of FIG. 5.

FIG. 11 is a back, side and plan perspective view of the trial implantof FIG. 7 inserted in the intervertebral space as an adjustable stop isadjusted.

FIG. 12 is a back, side and plan perspective view of the trial implantof FIG. 11 positively positioned in the intervertebral space.

FIG. 13 is a back, side and plan perspective view of the trial implantof FIG. 12 as the milling guide of FIG. 9 is mounted thereon.

FIG. 14 is a back, side and plan perspective view of the trial implantof FIG. 12 with the milling guide of FIG. 9 mounted thereon.

FIG. 15 is a back, side and plan perspective view showing the initialinsertion of a reamer into the milling guide of FIG. 9 mounted on thetrial implant of FIG. 12.

FIG. 16 is a back, side and plan perspective view showing the initialcutting using the reamer in the milling guide mounted on the trialimplant.

FIG. 17 is a back, side and plan perspective view showing the completecutting using the reamer where a stop thereon engages the milling guidemounted on the trial implant.

FIGS. 18-20 are cross sectional front, side and plan perspective viewsshowing the cutting action of the reamer in the milling guide.

FIG. 21 is an enlarged partially cross sectional front, side and planperspective view showing the keel cut made with the reamer using thecutting action depicted in FIGS. 18-20.

FIG. 22 is a back, side and plan perspective view of an alternativedesign where the proximal end of the milling guide has bearings.

FIG. 23 is a plan view of an alternative design of a trial implant headwith a through hole.

FIG. 24 is back, side and plan perspective view of an alternative designof a disposable pivot element.

FIG. 25 is a front, side and plan perspective view of the disposablepivot element depicted in FIG. 24.

FIG. 26 is a back, side and plan perspective view of another alternativemilling system with a pivot element having an elongated tube.

FIG. 27 is a front, side and plan perspective view of an alternativemilling guide used for drilling.

FIG. 28 is a front, side and plan perspective view of a modifiedembodiment of the alternative milling guide depicted in FIG. 27.

FIG. 29 is a back, side and plan perspective view of a handle attachedto the milling system of FIG. 5.

FIG. 30 is a front, side and plan perspective view of an alternatemilling guide used with a box chisel.

FIG. 31 is side elevation view of an adjustable bushing mounted on aportion of a reamer.

FIG. 32 is side elevation view of a bushing mounted on a portion of areamer together with spacing washers.

FIG. 33 is a back, side and plan perspective view of an alternative,taller pivot element to that shown in FIG. 25.

FIG. 34 is a front elevation view of an alternative milling guide tothat shown in FIG. 9 having two mill chambers on the upper part.

DETAILED DESCRIPTION OF THE INVENTION

The instruments and methods described herein are applicable forpreparing a wide range of artificial disc implants for insertion into anintervertebral space, typically for TDR (total disc replacement). Forthose instruments and methods described herein which include the conceptof forming cutouts to receive raised keels, the instruments and methodsdescribed herein are adaptable for use with any artificial disc implanthaving such keels. Thus, the depiction and description of the referencedimplant is exemplary.

With reference now to the drawings in which like numerals represent likeelements throughout the various views, it will initially be appreciatedthat the present invention is directed to improving the primarystability of an intervertebral implant 10, such as that disclosed U.S.Pat. No. 7,204,852 (Marnay et al.), which is located between adjacentvertebral bodies 12 (for fusion or non-fusion procedures) as shown inFIGS. 1-4. Implant 10 is designed with a keel 14 on both endplates 16contacting the adjacent vertebral bodies 12 as shown in FIG. 1. In orderto position implant 10 into the disc space provided after a discectomy,a cut needs to be made in the inferior as well as in the superiorvertebral bodies 12 to provide slots 18 as shown best in FIG. 2. Implant10 is shown partially between vertebral bodies 12 in FIG. 3, and thenfully inserted in FIG. 4.

The large majority of surgical techniques for TDR use chisels to performor prepare the keel cuts or slots 18. However, in accordance with thepresent invention, a drilling/milling system 20 has been developed as analternative in cases with hard bone and/or sclerotic endplates in orderto reduce the impact forces and in order to improve the cleaning of theposterior aspect of the keel cut. Milling system 20 includes thefollowing major instruments that interact with each other as broadlyshown in FIGS. 5-6: a trial implant 22 with an adjustable stop 24; adetachable handle 26 for trial implant 22 (shown only in FIG. 8); amilling guide 28; and a reamer 30 (which is used with existing/knownpower tool equipment).

Trial implant 22 is shown in greater detail in FIG. 7. It fulfills theknown function of determining a correct height of implant 10 to be used,for example where implant 10 comes in different heights, there will be adifferent trial implant 22 to accommodate each height (such as 5 mm, 6mm and 7 mm). A selected trial implant 22 having a one-piece implanthead 32 is thus inserted between the vertebral bodies 12 with the helpof detachable handle 26 to see if the selected trial head 32 fitsproperly, so that the correct implant height (and size, if desired) isthen known. Obviously, various heights of trial implants 22 withcorresponding heads 32 are available for such a trial and errordetermination; and if desired, different sized footprint and/or shapesof implants, etc. can also be tried if desired to determine the bestimplant 10 to be used. In accordance with the present invention, oncethe correct size of trial implant 22 is inserted, body 34 of trailimplant 22 then subsequently serves as base for milling guide 28 to beconnect to or to be mounted on.

As shown in FIG. 8, detachable handle 26 is removably attached (as by aball/détente interaction) to threaded shaft 36 of adjustable stop 24 forconvenience. It will be appreciated that adjustable stop 24 is bestshown in FIG. 7 as well and is movably mounted in the rear of body 34 oftrial implant 22 by threaded shaft 36. Adjustable stop 24 includes upperand lower vertebra engaging members 38 whose longitudinal positionrelative to trial head 32 is thus adjustable. Engaging members 38 areattached to shaft 36 and guided for movement in trial body 34 as shown,and engaging members 38 are positioned symmetrically relative to themidline of trial implant 22 for an improved resistance to A/P forces. Inaddition, the split design of engaging members 38 allows reamer 30 tomove in between the two engaging members 38, with engaging members 38offering additional soft tissue protection from reamer 30.Alternatively, trial implant 22 could include only one side stop memberor two stop members placed on either side of the upper and lowervertebral bodies 12. Trial implant 22 includes a central groove 40 intrial head 32 on the cranial (upper) side and on the caudal (lower) sideinto which reamer 30 can be moved or plunge into. Alternatively, trialbody 32 could include a cavity through the entire (split/forked) trialwith the same function.

Milling guide 28 is best shown in FIG. 9. Milling guide 28 is designedto connect to trial body 34 of trial implant 22. This connection occursas milling guide 28 is guided for movement along shaft 36 at the rear oftrial implant 22 and additionally guided by guidance feature 42 runningalong most of the length of trial implant 22 as best seen in FIG. 7.This guidance feature 42 is a combination of short flanges 44outstanding along either side (upper and lower) of trial body 34 andcorrespondingly spaced and shaped grooves 46 provided in the matingsurfaces of milling guide 28. Milling guide 28 is positioned on trialbody 34 until stop surfaces 48 mate up with the rear end of trial body34 which results in a fixed distance between the proximal end of millingguide 28 and the distal end of trial implant 22. Preferably, a lockingmechanism 50 is used to prevent milling guide 28 from unintentionallydisengaging from trial implant 22 and/or to eliminate any clearance/playbetween milling guide 28 and trial implant 22. Locking mechanism 50 isshown as mating wedges 52, but leaf springs, locking screws, or othermechanisms known in the art could be used. Milling guide 28 alsoincludes a handle attachment member 53 on one side to which a handle(such as handle 106 in FIG. 29) is removably attached as by a threadedconnection.

As shown in FIG. 10, milling guide 28 defines upper and lower chambers54 which guide respective reamers 30. Each chamber 54 is tapered fromfront to rear as shown best in FIGS. 18-20 to allow for a windshieldwiper milling cycle 60 (see arrow in FIG. 19, and compare the reamer 30position in FIGS. 18-20) around a pivot axis 56 located at the rear endof milling guide. In the embodiment shown, reamer 30 is restrictedlaterally, and is allowed milling in only a cranial-caudal direction.However, in alternate embodiments, chambers 54 could allow for millingin at least one other direction up to all directions. The millingfunction and technique is described in more detail hereafter. Further,chamber 54 could also be more cylindrical in nature to allow for a moretranslational milling cycle in cranial-caudal direction.

As an alternative to the disclosed embodiment, it will be appreciatedthat milling guide 28 could instead include only one reamer guidingchamber 54 which would be positioned on one (upper or lower) side oftrial implant 22. Then, after completing the first keel cut, reamer 30would then be retracted, the milling guide turned by 180° and reinsertedbefore milling the second (other side) keel cut. The reamer could alsobe pre-assembled to such a milling guide, to easily allow this millingguide to retract a certain distance before turning and reinsertion forthe other keel cut. As another alternative, the milling guide and thetrial implant could be designed as one instrument with the samefunctions described above.

As desired, different reamers 30 could be used with system 20 dependingon whether drilling and/or milling (side cutting) capabilities areprimarily needed. Exemplary reamers would thus include, for example,regular drills, Lindemann reamers, and cranial burrs; and other reamersas known and used in the art can also be used as desired. The cuttingend of reamer 30 is preferably conically shaped, with a smaller diameterat the distal (forward) end slowly expanding towards the bigger shaftdiameter. The benefit of the conical shape is that the smaller tipcompensates for the small play of the reamer within chamber 54 ofmilling guide 28. But alternatively, the reamer tip might becylindrical, tapered or a combination of cylindrical, tapered and/orconical as desired. Each reamer 30 also includes an integrated bushing58 which will come to rest against the back end of milling guide 30 tocontrol the depth of penetration of reamer 28 into vertebral body 12, inconjunction with the use of adjustable stop 24 as noted above. Whenbushing 58 comes to rest against the back end, it then acts as a bearingmember against the back end as described more fully below.

In use, drilling/milling system 20 is used in the following manner andwith reference to FIGS. 11-17. Initially, after performing thediscectomy (FIG. 1), the surgeon uses the trial implants to find thecorrect height (and footprint size if desired) of the implant 10 thatwill be needed for each particular vertebral space. As each, and moreimportantly, as the final or correct, trial implant is inserted with theintegrated adjustable stop 24 (FIG. 11), adjustable stop 24 not onlysecures trial implant 22 in its right position relative to the vertebralbodies 12 but it also assures that trial implant 22 will not slidefurther back into the spinal canal (FIG. 12). Once the right size andcorrect position as been found, milling guide 28 is mounted to trialbody 34 by sliding milling guide 28 over trial body 34 using guidancefeature 42 (FIG. 13) and locking milling guide 28 to trial body 34 withlocking mechanism 50 (FIG. 14).

Next, the surgeon performs the first cut on the vertebral body 12 of hischoice. Using a reamer or drill with side cutting capabilities, thesurgeon first drills/cuts straight into vertebral body 12 until bushing58 on reamer 30 is stopped by milling guide 28 as shown by FIGS. 15-17.Then, the surgeon sweeps in the direction of the endplate to completethe keel cut as shown in FIGS. 18-20. The surgeon could also start thecut by drilling/cutting along the endplate, and then sweeping into thevertebral body if desired; or drilling intermediate the two and sweepingboth up and down. Bushing 58 placed on reamer 30 avoids drilling/cuttingtoo deep into vertebral body 12, while acting as a bearing memberagainst the back end of milling guide 30. Depicted in FIG. 21 is anenlarge view of a keel cut or slot 18 made with system 20.

Finally, the surgeon removes reamer 30 and repeats the same operation onthe other vertebral body 12. FIG. 2 shows both keel cuts 18 as made bysystem 20 in the respective vertebral bodies 12. The cutting tool isconveniently powered by any known power tool, such as E-pen, MidasRex,Stryker TPS, etc. The first used reamer 30 could also be left in placeafter completing the cut or the first drilling hole to stabilize theconstruct while using a second reamer to mill the keel cut on theopposite side, as shown in FIG. 10 depicted both reamers 30. If the boneis extremely hard, reamer 30 could also be used as a drill several timesto weaken the bone before completing the keel cut with the sweepingmilling step. Bushing 58 acting as a stop on reamer 30 couldalternatively be detachable; or adjustable to allow for differentdrilling/milling depth, as shown by bushing 58′ in FIG. 31 which isadjustable by disengagement of a simple set screw 59 or the like.

It will be noted that milling guide 28 provides tapered mill chambers 54which allow the reamer to pivot about the proximal end of the guide asshown in FIG. 18-20. For this reason, the proximal (rearward) end 64 ofmilling guide 28 is slightly curved towards the cranial and caudal ends.This allows reamer 30 to drop slightly deeper when angling towards trialbody 32, resulting in a straighter wall at the posterior end of keel cut18 instead of an arc as would be expected from such a pivoting motion.Alternatively, the proximal end of milling guide 28 could also bestraight if such an arced end is not objectionable. It will also beappreciated that the slimness and shape of system 20 also allows goodvisibility for the surgeon.

Depicted in FIG. 22 is an alternate design of a proximal end 66 ofmilling guide 28 in which a pivot element 68 with a sleeve bearing 70therein is provided in proximal end 66 for each mill chamber 54. Sleevebearing 70 receives the shaft of reamer 30 in order to minimize frictionbetween milling guide 28 and reamer 30. Sleeve bearing 70 isconveniently supported by pivot elements 72 rotating around small pins(not shown) which allow a controlled sweeping motion of reamer 30.

Depicted in FIG. 23 is an alternative embodiment of a trial implant head76 with a through hole 78 therein. Through hole 78 allows the necessaryroom for reamers 30, and strength of trial head 76 is not compromisedsince the forward part of trial head 76 is closed as shown. Through hole78 is provided to improve the retention in trial head 76 of the cut bonematerial created during the reaming process.

Depicted in FIGS. 24-25 is an alternative disposable pivot element 80similar to pivot element 68 described above. If the life span of thesleeve bearing therein (not shown) is considered too short or reuse isnot desired, then pivot element 80 supporting the sleeve bearing is madedisposable. The sleeve bearing and pivot element 80 would then bereplaced in the milling guide after each surgery. The material used forthis type of pivot element could be PEEK. A pair snap-on spring-likereceiving feet 82 provided on pivoting element 80 allows pivot element80 to be attached to and detached from pins 84 (see FIG. 22) at the rearend of the milling guide.

Alternatively, pivot element 80 could be made in different heights, suchas shown by pivot element 80′ in FIG. 33 which is taller than pivotelement 80 due to the height of head 86 (though alternatively, theportion below head 86 could instead be heightened). With differentheights of pivot elements, the surgeon would select the height desiredto position the cutting end of reamer 30 relative to where bushing 58contacts the pivot element as needed. Such pivot elements 80′ may or maynot be designed to be disposable.

Depicted in FIG. 26 is another alternative embodiment of a millingsystem 20′ having a pivot element 88 including an elongated tube 90designed to provide guidance for reamer 30. Elongated tube 90 extendsfrom the bearing/pivot portion at mounting pins 92 to the distal(forward) end of the milling guide to provide enhanced support forreamer 30 if needed or desired. As another alternative, one or more thinspacing washers 62 a and/or 62 b as depicted in FIG. 32 could be addedas needed below bushing 58. Washers 62 are used to space bushing 58slightly further from pivot element 88 (or from milling guide 28 inother embodiments noted above), and thus would become part of thebearing member for reamer 30 against the back end of milling guide 28.Washers 62 a and 62 b have different heights as shown, and one or moreof each, or others of different heights, could be used as desired. Suchwashers would preferably slide frictionally along reamer 30 so thatwashers 62 would not move along reamer 30 without being positivelymoved, and hence would not fall off of reamer 30 accidentally.

Depicted in FIG. 27 is an alternative embodiment of a milling guide 96for straight hole drilling or cutting. Milling guide 96 is provided asan option when it is desired to reduce the impact forces for asubsequent chiseling step used to form the keel cut as typical in theprior art. The keel cutting technique would include drilling of one ortwo parallel straight holes per keel using a drill 98 to remove bonebefore using either a wedged and/or box chisel (not shown). For thispurpose, milling guide 96 includes two guide holes 100 a and 100 b foreach keel cut location. The system shown also has an alternativeadjustable stop design having two side stops 102 positionedsymmetrically relative to the insertion or longitudinal axis. In orderto remove more bone of the vertebral endplates, hole 102 b in millingguide 96 could be angled towards the trial body as shown in FIG. 28 withmilling guide 96′. If reamer 30 has a tapered/conical tip, the resultingdrilled hole caused by the tapered/conical tip would be designed to beparallel to the trial surface, or at least allow for more bone removalat the proximal (forward) end of the trial body. In the figure, thesurgeon would drill two holes on each side, but the two holes on eachside would not be parallel.

If desired, any of the milling (or drilling) guides is stabilized orcontrolled with a guide handle 106 as shown in FIG. 29 (and alsopartially in FIG. 22). Alternatively, guide handle 106 could also beattached to a retainer or a retractor system such as the SYNFRAME® bySynthes.

If the surgeon feels comfortable with chiseling to perform the keel cut,trial implant 22 can also accommodate a guide 110 for a box chisel 112as shown in FIG. 30. Guide 110 is mounted similarly to milling guide 28,allowing the surgeon to use either a drill or a chisel method to performthe keel cut as desired.

While the above embodiments have been depicted where an upper and lowerkeel slot is made in the adjacent vertebral bodies 12, there may besituations where an implant has side by side or dual (or more) keels onone (or both) sides, so that cutting of two keel slots is desired in avertebral body 12. In such situations, it would be possible to provide amilling guide 28′ as shown in FIG. 34. Milling guide 28′ has two side byside mill chambers 54′ on the top portion, with a corresponding rear end(not shown) for accommodating a reamer 30 in each mill chamber 54′. Atrial implant (not shown) which accommodates the two mill chambers withtwin trial grooves would thus also be provided.

While the components described above are preferably made out of metalssuch as stainless steel, titanium or titanium alloy, alternatively somecomponents could be made out of composites or polymers. In addition,this type of bone cut procedure is not limited to the cervical spine,but could be used any where in the human body and in particular it couldbe applied for Lumbar TDR.

Although the invention has been described in considerable detail withrespect to preferred embodiments thereof, it will be apparent that theinvention is capable of numerous modifications and variations, apparentto those skilled in the art.

1. An instrument system, comprising: a trial head sized to be receivedin an intervertebral space; a milling guide configured to be supportedrelative to the trial head, said milling guide including: a millingguide body that has a proximal end and a distal end that is spaced fromthe proximal end along a first direction, wherein the milling guidedefines a first chamber and a second chamber, the first chamber iselongate along the first direction, and the first and second chambersare arranged side by side with respect to each other; and a pivotelement pivotally coupled to the milling guide body, the pivot elementconfigured to pivot relative to the second chamber about a pivot axisthat extends in a second direction that is substantially transverse tothe first direction; and a tool configured to be at least partiallyreceived in the pivot element so as to extend through the first chamber,such that when the pivot element pivots about the pivot axis while thetool is at least partially received in the pivot element and extendsthrough the first chamber, the tool creates a channel in a vertebrawhen 1) the trial head is received in the intervertebral space, and 2)the milling guide is supported relative to the trial head; wherein eachof the first and second chambers is configured to receive the tool. 2.The instrument system as claimed in claim 1, wherein said first chamberis tapered in a cranial to caudal direction.
 3. The instrument system asclaimed in claim 2, wherein the first chamber defines a cross-sectionaldimension that increases along a direction from the proximal end towardthe distal end.
 4. The instrument system as claimed in claim 1, whereinsaid tool comprises a cutting head which is conically shaped.
 5. Theinstrument system as claimed in claim 4, wherein said trial head definesa longitudinal groove, and said longitudinal groove is configured toreceive said cutting head.
 6. The instrument system as claimed in claim5, wherein said longitudinal groove is larger than said cutting head toprovide a repository for cut vertebra.
 7. The instrument system asclaimed in claim 1, further comprising a trial implant that comprisesthe trial head, and an adjustable stop configured to contact at leastone vertebra when said trial head is properly positioned in theintervertebral space, the adjustable stop configured to move relative tothe trial head.
 8. The instrument system as claimed in claim 1, whereinthe proximal end of said milling guide body comprises a bearingmechanism that defines said pivot axis when the tool is at leastpartially disposed in the pivot element.
 9. The instrument system asclaimed in claim 8, wherein said bearing mechanism is a curvature of aportion of said proximal end of said milling guide body.
 10. Theinstrument system as claimed in claim 8, wherein said bearing mechanismis a movable bearing mounted in said proximal end of said milling guidebody.
 11. The instrument system as claimed in claim 10, wherein saidmovable bearing is removably mounted in said proximal end.
 12. Theinstrument system as claimed in claim 8, wherein said bearing mechanismcomprises an elongate tube that extends in a direction toward saiddistal end, the elongated tube configured to support the tool.
 13. Theinstrument system as claimed in claim 1, wherein the tool comprises abearing member that is configured to engage a portion of the proximalend, and the proximal end is configured to define the pivot axis whenthe bearing member engages the portion of the proximal end so as toallow the tool to pivot about the pivot axis with respect to the millingguide body.
 14. The instrument system as claimed in claim 1, furthercomprising a support member configured to be connected between the trialhead and the milling guide.
 15. The instrument system as claimed inclaim 14, wherein the support member is configured to carry the trialhead as the trial head is inserted into the intervertebral space. 16.The instrument system as claimed in claim 14, wherein the support memberis a trial body.
 17. The instrument system as claimed in claim 1,wherein at least a portion of the first chamber extends through themilling guide body.
 18. The instrument system as claimed in claim 1,wherein the intervertebral space is defined between first and secondvertebral bodies that are spaced in a cranial to caudal direction, andthe second direction is further substantially transverse to the cranialto caudal direction.
 19. The instrument system as claimed in claim 18,further comprising a pivot pin pivotally coupling the milling guide bodyto the pivot element, wherein the pivot pin defines the pivot axis. 20.An instrument system, comprising: a trial head sized to be received inan intervertebral space; a milling guide configured to be supportedrelative to the trial head, said milling guide including: a millingguide body that has a proximal end and a distal end that is spaced fromthe proximal end along a first direction; and an upper pivot elementpivotally coupled to the milling guide body, a lower pivot elementpivotally coupled to the milling guide body, the lower pivot elementspaced from the upper pivot element along a second direction that issubstantially transverse to the first direction, wherein each of theupper and lower pivot elements is configured to pivot relative to themilling guide body about respective upper and lower pivot axes thatextend in a third direction that is substantially transverse to thefirst and second directions; and a tool configured to be at leastpartially received in at least one of said upper or lower pivot elementsso as to pivot about at least one of the respective upper or lower pivotaxes when the at least one of said upper or lower pivot elements pivotsabout the respective upper or lower pivot axis, such that the tool isconfigured to create a channel in a vertebra when the tool is at leastpartially received in the at least one of the upper or lower pivotelements and the at least one of the upper or lower pivot elementspivots about the respective upper or lower pivot axis.
 21. Theinstrument system as claimed in claim 20, wherein the milling guidefurther comprises an upper chamber and a lower chamber that is spacedfrom the upper chamber along the second direction, each of the upperchamber and the lower chamber is elongate along the first direction, andeach of the upper chamber and the lower chamber is configured to receiveat least a portion of the tool.
 22. The instrument system as claimed inclaim 21, wherein at least one of said upper and lower chambers istapered in a cranial to caudal direction.
 23. The instrument system asclaimed in claim 22, wherein said tool has a cutting head that isconically shaped.
 24. The instrument system as claimed in claim 23,wherein said trial head defines an upper and a lower longitudinalgroove, and at least one of the upper or lower longitudinal grooves isconfigured to receive said cutting head.
 25. The instrument system asclaimed in claim 24, wherein at least one of said longitudinal groovesdefines a through hole that is wider than said cutting head to provide arepository for cut vertebra.
 26. The instrument system as claimed inclaim 22, wherein each of the upper and lower chambers defines arespective cross-sectional dimension that increases along a directionfrom the proximal end toward the distal end.
 27. The instrument systemas claimed in claim 21, wherein each of the upper and lower chamberextend through the milling guide body.
 28. The instrument system asclaimed in claim 27, further comprising a lower pivot pin pivotallycoupling the milling guide body to the lower pivot element, wherein thelower pivot pin defines the lower pivot axis.
 29. The instrument systemas claimed in claim 20, further comprising a trial implant thatcomprises the trial head, and an adjustable stop that is configured tocontact at least one vertebra when said trial head is properlypositioned in the intervertebral space, the adjustable stop configuredto move relative to the trial head.
 30. The instrument system as claimedin claim 20, wherein the proximal end comprises at least one bearingmechanism that is configured to define the upper or lower pivot axiswhen the tool is at least partially disposed in the at least one of theupper or lower pivot elements and when the portion of the tool engagesthe proximal end so as to allow the tool to pivot about the upper orlower pivot axis with respect to the milling guide body.
 31. Theinstrument system as claimed in claim 30, wherein the at least onebearing mechanism is a curvature of a portion of said proximal end ofsaid milling guide body.
 32. The instrument system as claimed in claim30, wherein the at least one bearing mechanism is a movable bearingmounted in said proximal end of said milling guide body.
 33. Theinstrument system as claimed in claim 32, wherein said movable bearingis removably mounted in said proximal end.
 34. The instrument system asclaimed in claim 30, wherein the at least one bearing mechanismcomprises an elongate tube that is elongate along the first direction,and the at least one bearing mechanism is configured to support thetool.
 35. The instrument system as claimed in claim 20, furthercomprising a support member configured to be connected between the trialhead and the milling guide.
 36. The instrument system as claimed inclaim 35, wherein said upper and lower chambers are arranged side byside with respect to each other.
 37. The instrument system as claimed inclaim 35, wherein the support member is configured to carry the trialhead as the trial head is inserted into the intervertebral space. 38.The instrument system as claimed in claim 35, wherein the support memberis a trial body.
 39. The instrument system as claimed in claim 20,wherein the tool comprises a bearing member that is configured to engagethe proximal end, and the proximal end configured to define the upper orlower pivot axis when the bearing member engages the proximal end so asto allow the tool to pivot about the upper or lower pivot axis withrespect to the milling guide body.
 40. The instrument system as claimedin claim 20, further comprising an upper pivot pin pivotally couplingthe milling guide body to the upper pivot element, wherein the upperpivot pin defines the upper pivot axis.